Sure, making a dynamic language fast is not easy. However, back then people thought LLVM was a magic wand. It wasn't. There were several problems.
First, the Unladen Swallow team (IIRC) spent a lot of time fixing bugs in LLVM.
Second, LLVM isn't fast at compiling code, at least not for a JIT. This is legitimately surprising, because the official LLVM Tutorial implements a JIT.
Third, LLVM used to stand for Low Level Virtual Machine. I don't know when it stopped standing for that; clearly it hasn't for a long time. But with “Virtual Machine” in the title, you can see why people might have thought it would be suitable for implementing a dynamic language. cf. GraalVM these days.
Exactly. LLVM and Cranelift are essentially code generation backends when you’re applying them for dynamic languages. You need an entire actual custom compiler in front of them to get good code out of them.
Unladen Swallow is seared in my mind. It was a failed project, back in 2009, to improve the speed of CPython by using LLVM as a JIT.